Add schema for INGRID user config files

[bryan-garcia]

This package is quite useful for the validation of user config files, and I've found it quite handy in other projects.

We should add a validation step that leverages this tool rather than rely on the brute-force dictionary checks we have.

[Plasdom]

I think adding a schema would be really useful for new users. For people using VS Code, a dependency-free option is to let it do the validation (see here). This adds code completion and linting too.